1. Field of the Invention
The present invention relates to the production of metal chlorides by the fluidised bed chlorination of oxidic materials.
2. Brief Description of the Prior Art
Metal chlorides may be used in the metallurgical or pigment industries and for such use may be produced from iron-containing oxidic ores or deposits by an initial beneficiation to remove iron selectively to produce a concentrate having an increased concentration of a desired constituent or constituents followed by the substantially non-selective fluidised bed chlorination of the concentrate to produce a gaseous effluent rich in the chloride of the desired constituent or constituents and containing restricted quantities of chlorides of other chlorinatable ore constituents, if and to the extent present. Examples of beneficiation processes are the electrosmelting of low grade titanium dioxide-containing deposits to produce a titanium dioxide rich slag, which may have a content of titanium dioxide as high as 85% by weight or even higher, and the chlorination beneficiation of ore such as ilmenite or chromite to produce a concentrate which may contain a similar content of titanium dioxide or up to, for example, 60% by weight of chromic oxide.
While the iron content of an ore or deposit may be reduced successfully by beneficiation processes the concentrate so produced usually still contains an appreciable quantity of minor constituents which have not been removed with the iron which give rise to difficulties in conducting a subsequent fluidised bed chlorination step. Certain metal oxides usually present in small but appreciable proportions in oxidic ores or deposits chlorinate to give low melting point chlorides having a low vapour pressure with the result that, at usual chlorination temperatures of about 800.degree. C. to 1100.degree. C., they remain present in the bed in the liquid form and, as the chlorination progresses, gradually build-up in content in the bed and at some point cause bed agglomeration and cessation of the process. More specifically magnesium, manganese and calcium chlorides, the oxides of some or all of which are usually present as minor constituents in oxidic ores, all have melting points below 800.degree. C. and very low vapour pressures. At 800.degree. C., the maximum vapour partial pressure of manganese chloride is below 0.03, of magnesium chloride is below 0.003 and of calcium chloride is below 0.00002 atmospheres. It can be seen from this that even a small content of these chlorides can give rise to problems particularly in continuously operated processes. In some cases a desired ore constituent may itself give rise to similar difficulties. Chromium chloride for example can build up in quantity in a fluidised bed in which chromite is being chlorinated and contribute to the bed agglomeration problems as well as providing a problem of recovery. At 800.degree. C. the maximum partial vapour pressure of chromium chloride is 0.033 atmospheres. At higher temperatures the vapour pressure is higher but nevertheless the problem of retention of chromium chloride in the bed persists. Also other metal chlorides are relatively involatile and can give rise to problems due to accumulation in the bed in the course of the chlorination of an ore or concentrate containing it as a minor constituent.
Many beneficiation processes do not achieve 100% separation of the iron content of ores or ore deposits but may leave up to 5% or even up to 10% or 15% by weight of the ore or deposit in the form of iron oxide. This iron oxide would form a usually undesired constituent of the product of the further chlorination of such a beneficiation. This applies also to certain ores which may in their natural state already be so rich in a desired metal oxide constituent other than iron as to make a separate beneficiation process uneconomic. It is well known that, while ferric chloride has a relatively high vapour pressure and does not present undue problems with regard to fluidised bed chlorination processes ferrous chloride, which is often formed in varying proportions with ferric chloride is subject to many of the problems encountered as a result of the presence of magnesium, manganese or calcium chlorides. Ferrous chloride melts at below 700.degree. C. and at 800.degree. C. has a maximum partial vapour pressure of about 0.08 atmospheres.
In the course of a fluidised bed chlorination in the presence of carbon it is thought that, initially, the carbon may absorb liquid chlorides present in the bed enabling fluidisation to proceed unimpeded for a time, but that the capacity of the carbon to continue to do this then diminishes and that the resulting presence of unabsorbed liquid chlorides in the bed encourages the retention in the bed of chlorides, for example chromium chloride, which would normally be expected to be substantially in the gaseous phase at the bed temperature prevailing. While the above is a theory to which the Applicants do not bind themselves, it does correspond to the observed behavour of fluidised bed processes for the chlorination of materials containing the constituents in question to proceed normally for a time and then to become agglomerated relatively suddenly as if a critical threshhold in the content of substances deleterious to the fluidised state had been reached and exceeded.
Against the above background it can be understood why the fluidised bed chlorination of titanium dioxide slag, which may typically contain about 5-15% Fe.sub.2 O.sub.3, 0.5-3% MnO, 0.5-5% MgO and up to 0.5% CaO has been found to be difficult. In fact titanium dioxide slag is not generally regarded as chlorinatable on its own on a practicable industrial basis. This problem is acknowledged in Australian Patent No. 237857 which describes a prior proposal to alleviate the problem by withdrawing a large portion of the bed material from the zone of chlorination, washing the bed material to remove fused chlorides and returning the washed residue to the bed. This prior proposal entails cooling the withdrawn portion of the bed to enable it to be washed and reheating it separately to the desired chlorination temperature. This is not practicable in commercial operation in view of the very large heat losses involved. Australian Patent No. 237857 offers as a solution to the problem the use of a specially designed chlorination fluidised bed reactor in which the fluidising chlorine is introduced at a point above the bottom of the fluidised bed and in which there is provision for purging a small portion of the bed material from a point below the point of introduction of the fluidising chlorine in which section of the reactor agglomerated bed particles tend to gather. The amount of the purge of bed materials may be up to 15% of the ore fed to the bed and in one Example the residual titanium content of the purge material was 14% corresponding to a TiO.sub.2 content of over 23%. In practice this represents an unacceptable loss of the desired titanium chloride product. A chromite beneficiate may, typically, contain as much as 10-25% Al.sub.2 O.sub.3 and 5-12% MgO so that, despite a possibly very low content of CaO and MnO, similar problems apply in addition to the problem of retention of chromium chloride in the bed as described above.
The present invention is intended to alleviate, at least partly, the problems discussed above arising from the presence of minor constituents. It may be applied to any ore, ore deposit or ore concentrate of which a desired constituent is chlorinatable to give a vaporous chloride at a temperature above about 800.degree. C. for example from 800.degree. C. to 1100.degree. C. and which contains any appreciable quantity of the relevant constituents for example, without intending limitation to such a minimum quantity, at least 0.2% by weight of one or more of said constituents, with the proviso that calcium oxide should not be present, due to its particularly low vapour pressure, in more than 0.5% by weight. The above references to titanium dioxide slag and chromite beneficiate are, of course, illustrative only.